1. Field of the Invention
The present invention relates to a brake control system employing a fluid pressure control valve for controlling a brake fluid pressure distribution between front and rear wheels, and particularly to a system for preventing a rear-wheel lock in an automotive vehicle during braking.
2. Description of the Prior Disclosure
Recently, there have been developed and disclosed various brake control systems provided in conjunction with a hydraulic type brake system in an automotive vehicle for preventing a wheel-lock during braking. An anti-skid brake control system is well known as such a wheel-lock preventing device. The anti-skid brake controls systems reliably control braking forces applied to vehicle wheels independently of each other or commonly so as to prevent a wheel-lock. However, since the anti-skid brake control system is relatively expensive, the diffusion of the anti-skid brake control system is low.
Alternatively, a fluid pressure control valve employed in a fluid pressure operated brake system, such as a hydraulic type brake system is traditionally utilized as a conventional wheel-lock preventing device. The fluid pressure control valve operates to vary the brake fluid pressure distribution between front and rear wheels. As is generally known, during braking, more of the car weight is transferred to the front wheels and thus the car's weight becomes less at the rear wheels. If normal braking were continued with the front-wheel brake fluid pressure equal to the rear-wheel brake fluid pressure, the brakes could first lock the rear wheels so that the rear tires skid. As a result, the rear wheels could throw the entire car into a rear-end skid. This could result in oversteer and/or spinning on a low frictional road surface, such as wet or icy roads. For this reason, the previously noted fluid pressure control valve is employed in conjunction with the hydraulic type brake system to provide an advantageous braking action according to which front-wheel lock gets priority over rear-wheel lock so as to eliminate oversteer tendencies on turns. As indicated by a broken line of FIG. 4, it is desired that both front and rear wheel brake fluid pressures are varied in accordance with an ideal brake fluid pressure distribution characteristic curve wherein both front and rear wheels are locked simultaneously so as to provide an optimal braking efficiency.
In view of the above, as seen in a solid line corresponding to a brake fluid pressure characteristic curve a-b-c in FIG. 4, the aforementioned conventional fluid pressure control valve controls the brake fluid pressure distribution between front and rear wheels such that the rear wheel-cylinder brake fluid pressure P.sub.R is set essentially at the same value as the front wheel-cylinder brake fluid pressure P.sub.F in a relatively low fluid pressure range from a to b and held substantially constant at a lower level than the front fluid pressure in a relatively high fluid pressure range from b to c. The point b is in general referred to as a "split point". A fluid pressure P.sub.S0 corresponding to the split point b is referred to as a "critical brake fluid pressure". Particularly, the critical fluid pressure P.sub.S0 will be referred to as a "reference critical brake fluid pressure" wherein the front wheel-cylinder brake fluid pressure P.sub.F is equal to the rear wheel-cylinder fluid pressure P.sub.R and both front and rear wheels are simultaneously locked. Such a brake fluid pressure distribution achieved by the conventional fluid pressure control valve, eliminates oversteer due to a rear-wheel lock created in a relatively high fluid pressure range above the split point b. In other words, the brake fluid pressures in the front and rear wheel-cylinders are set such that the front wheel lock gets priority to the rear wheel lock to avoid oversteer tendencies of the vehicle during braking on turns.
However, in the prior-art brake control system employing the previously noted conventional fluid pressure control valve achieving a braking control according to the brake fluid pressure characteristic curve a-b-c, the rear-wheel is finally locked after the front-wheel is first locked, during quick braking or hard braking on a low frictional road surface, such as wet or icy roads, even when a rear-wheel brake fluid pressure control is achieved by the fluid pressure control valve. During quick braking on a low frictional road surface, since the vehicle wheels become all locked, a braking distance would be increased.